Spark plug and the manufacture thereof



1946- D. H. CORBIN SPARK PLUG AND THE MANUFACTURE THEREOF Filed Dec. 8,1943 r g VQZ w N 502g 7 6W Bnoentor attorney- Patented May 28, 1946SPARK PLUG AND THE MANUFACTURE THEREOF Douglas Haynes Corbin, signor toGeneral Mot Mich., a corporation Application December 8, 1943 ,=SerialNo.

Dunstable, England, asors Corporation, Detroit, v I Delaware In GreatBritain December 12, 1942 Claims. (01. 123-w-169) This invention relatesto spark plugs for internal combustion engines.

In the specification of my prior Letters Patent No. 2,162,062 grantedJune 13, 1939, there is described and claimed a method of making a sparkplug by positioning in the insulator a spark electrode with the centralbore, introducing ametal for constituting the core of the electrode intothe bore and bringing it into contact with the said electrode tip whilstin a molten state and then letting the metal cool so as to retain theelectrode in position.

In practice the spark electrode tip may be constituted of a platinumalloy and the core of silver so that there is a minimum of corrosion anderosion and good heat conductivity from the hot tip of the plug.

It has also been proposed in Patent No. 2,106,578 granted January 25,1938, to Schwartzwalder and Kirk and in Patent No. 2,248,415 grantedJuly 8, 1941, to Schwartzwalder and Rulka to seal an electricalconductor in a spark eiiected a gas-tight connection between theconductor and the insulator. According to the preferred embodiment ofthe invention disclosed in the second of the above patents, the glassseal consists of a mixtureof glass and copper. Nevertheless even withthis latter seal it was found that there was a tendency for highresistances to develop during service and it was conjectured that thismight be due to the production of a small deposit of silver oxide thesilver core due to the fact that on cooling the silver column tended toshrink away from the seal leaving in some cases a very small areabetween the head of the silver core during service a small spark wouldbe produced with the consequent formation of ozone which at the normalworking temperature would rapidly produce a non-conductive silver oxide.

The present invention has for one of its objects to overcome the defectreferredto.

According to the invention, an improved spark plug manufactured ahereinbefore described is tip so'as to be in communication between theseal and 7 diameter than the diameter trical connection between the coreof the electrode and the seal. Another object of the invention consistsin improving cast metal electrodes of the class described. It has beenfound that when the metal is cast into a smooth-walled bore there issome tendency for the metal to pull itself apart and to fracture at somephase of its setting, due to shrinkage. I have found that this tendencymay be overcome by corrugating the bore, preferably spirally, wherebythe accommodation of the silver core to the corrugations results in theshrinkage strains being spread evenly through the core instead ofaccumulating at any one place and causing fracture.

Figure 1 is a vertical insulator having a corrugated bore as hereinafterdescribed.

Figure 2 is a view similar to Figure 1 but with the electrode in placein the bore.

Figure 3 is a front elevation of the electrode disclosed in sectioninFigure 2.

Figure 4 is a vertical section through a spark plug showing a modifiedform of electrode.

Referring first to the construction shown in Figures 1 to 3, theinsulator 4 is provided with of three different diamences la, 4b, 4c.

The bore 4a which is of serves to accommodate the platinum alloy sparktip 5 which is provided with a head 5:: of greater of the bore in sothat the said head So when resting on the shoulder 4d intermediate thebores la and ib correctly positions the said tip 5 relatively to theinsulator 4.

The wall of the bore 4b is formed with corrugations arranged in a spiralform.

The tip 5 is secured in position by a silver core 6 which is introducedin the form of a length of silver rod which is rendered molten, thensubthe smallest diameter jected to the influence of pressure andsubsequently cooled so that the said core 6 is molded to the walls ofthe bores 4b and 4c and caused to secure the spark tip within theinsulator as described in the said prior Patent No. 2,162,062.

It is found that as the molten silver in setting tlons results in theshrinkage strains being spread section of a spark plug rugation at anyone place as was possible heretofore in the case of a core cast in asmooth bore. Furthermore, a secure fixing of the central electrodewithin the insulator I is insured by the accommodation of the core tothe corrugations.

The corrugation of the section lb also assists in the manufacture of theinsulator I since this may be molded around a central core which cansubsequently be removed by unscrewing the core from the completedinsulator.

In the modification shown in Figure 4 the corof the lower portion of thebore has been omitted to illustrate the prior smooth bore designpreviously referred to. This figure illustrates the second feature of myinvention which consists in the provision of a spring about which themolten electrode metal is cast and the glass seal is fused to provide agood conducting path for the passage of current from the one to theother. This feature of my invention may obviously be employed whetherthe bore in the insulator is smooth or corrugated.

As in the embodiment of my invention, previously described, the headedspark tip 2 is inserted into the bore of the insulator 3 so that thehead 2a rests on the shoulder 3a. A length of silver rod is thenintroduced into the bore followed by a small metallic coil spring 5. Theassembly is first heated to render the silver molten and then is rotatedrapidly and subsequently cooled so that the silver accommodates itselfto the bore of,

the insulator 3 in the manner described in pending patent applicationSerial No. 508,549 filed November 1, 1943,

After cooling, the lower portion of the metallic coil spring 5' is foundto be embedded n the adjacent portion of the silver core I.

A gas-tight sealing of the silver core is effected by inserting in thebore of the insulator 3 a pellet of glass and pulverized copper which isrendered plastic and subjected to pressure so that on cooling itconstitutes a conductive seal 6' as described in the prior Patent No.2,248,415.

The formation of the seal 6' as aforesaid results in the upper portionof the spring 5 becoming embedded in the seal Ii.

As illustrated, it will be seen that the spring 5' does not extendcompletely through the seal 6' and consequently it will be understoodthat the seal 6' serves as the conducting medium between the core I andthe upper portion of the center electrode, not shown.

If desired, however, the spring 5' can be permitted to extend completelythrough the seal 6' so as to be capable of a metallic contact with theupper part of the electrode.

It is found in practice that by utilizing a coil spring 5' there is notendency for high resistances to develop when a plug is in service duepossibly to the formation of silver oxide in the minute gap which mayobtain between the seal 5 and the core 1 owing to shrinkage on cooling,and it is considered that the efllcacy of the coil spring 5' isoccasioned by the fact that the spring 5' not only bridges any gap whichmight be formed upon the shrinking of the silver core, but that in viewof the very small angle at which the spring 5 enters the seal 6' anysubsequent shrinkage movements would result in the pulling away of avery small piece of the seal 6' rather than the fracture of the spring5'.

I claim:

1. The combination of a spark plug having a bore therein and anelectrode in the bore comprising a section of metal cast in the bore, aconducting glass seal fused in the bore and overlying said cast metalsection, and a spring embedded in the said section and seal and formingan electrically conductive connection therebetween,

2. In the combination as defined in claim 1, said spring being of spiralform and having its axis running lengthwise of the bore.

3. The combination of a spark plug having a bore therein, portions ofsaid bore being of different diameter providing a shoulder therebetween,a spring resting on said shoulder, a metal electrode cast in the smallerportion of said bore and surrounding and embedding a portion of thespring, and a conducting glass seal fused in the upper portion of saidbore, overlying said elec- .trode and surrounding and embedding at leasta portion of said spring so that the spring forms an electricallyconductive connection therebetween.

4. The combination of a spark plug having a bore therein and anelectrode in the bore comprising a section of metal cast in the bore, aconducting glass seal fused in the bore and overlying said cast metalsection, and a spring embedded in the said section and seal and forminan electrically conductive connection therebetween, a portion of thebore in which said metal section is cast being transversely corrugatedto interlock with the cast metal.

5. The combination of a spark plug having a bore therein, portions ofsaid bore being of different diameter providing a shoulder therebetween,a spring resting on said shoulder, a metal electrode cast in the smallerportion of said bore and surrounding and embedding a portion of thespring, and a conducting glass seal fused in the

